Attoclock turns electrons into movie stars

AN ELECTRON takes just billionths of a billionth of a second to escape its host molecule - mere attoseconds. Now we have the first snapshots of what is the initial step in almost every chemical reaction.

"We can watch not only the atoms and the nuclei in a chemical reaction. Now we can even watch the electrons," says physicist Andreas Becker of the University of Colorado in Boulder. His team, mostly based at Goethe University in Frankfurt, Germany, zapped a molecular hydrogen ion - composed of two protons and one electron, the simplest molecule known - with an ultrafast infrared laser pulse. This booted the electron out of the ion and allowed the researchers to trace the path it took.

It's already possible to make movies of molecules and atoms in motion during chemical reactions using laser strobe lights that flash every few femtoseconds (10-15 seconds). However, before two substances can react, an electron must first make the leap from one atom or ion to another, a process that takes only a matter of attoseconds (10-18 seconds).

Laser pulses of 80 attoseconds duration exist and in theory could be used to make a movie of an electron's motion. Becker's team used laser pulses lasting femtoseconds, which are easier to produce. By rotating the pulses once every 1.55 femtoseconds, their pulses tracked motion every 15 to 20 attoseconds, similar to the way the minute hand on a clock tracks minutes, even though it takes an hour to complete one cycle.

The team expected the electron to break free of the hydrogen ion when the laser's electric field was strongest. Surprisingly, the electron made its escape (see the white arrow) 350 attoseconds later. "It changed our understanding of how a molecule is ionised," says Becker. "We thought we understood this process, at least for the simplest molecule." The findings will be published in Physical Review Letters.

More work is needed to determine what causes the electron's delay, a first step towards precisely controlling reactions via electron motion, says Becker. "You would like to take an electron and push it where you wish it to go," says attosecond pioneer Paul Corkum of the University of Ottawa in Canada. "That's the ultimate dream."

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The Steve McQueen of the subatomic world (Image: Jila, University of Colorado)